RE: Pressure is not constant in NPT ensemble

From: Nikolaos Glykos (
Date: Sun Aug 01 2010 - 15:37:17 CDT

Search the archives. It has been discussed over-and-over again. You
could even predict the magnitude of the fluctuations expected for your

----- Μήνυμα από ---------
     Ημερομηνία: Sun, 1 Aug 2010 14:31:11 -0400
            Από: Parisa Akhski <>
           Θέμα: RE: namd-l: Pressure is not constant in NPT ensemble
           Προς: Ajasja Ljubetic <>

> Hi,
> I am just wondering how much deviation form the target pressure is
> allowed in NPT ensemble by NAMD when using "LANGEVIN PISTON PRESSURE"?
> According to the
> it seems that in order to check the reliability of the pressure, we
> need to run a constant volume cutoff simulation in a cell that is
> larger than the molecular system by at least the cutoff distance;
> so, the pressure for this isolated system should average to zero
> over time. I set the pressure on 1bar and in the output file it
> starts form a big negative value and goes up but fluctuates between
> -50 and 50 bar (does not get zero finally). Is this OK or does
> anyone have any suggestion?
> Thanks,
> Parisa

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